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each complete computation moves the atmosphere forward into the future only a few minutes.

      And so the whole process starts all over again. The computer begins analyzing the data in each block with the new values it has forecast, nudging the weather system forward a few minutes at a time until the desired length of weather forecast is achieved. The experts say that to produce a weather forecast of a few days, modern computer models may need to complete more than one trillion calculations (1,000,000,000,000). So just how super are these computers? Scientists figure they are about six million times more powerful than the average desktop computer.

      The forecast method known as Numerical Weather Prediction has led to some pretty impressive improvements in recent years, but nobody expects computer models to make individual human forecasters obsolete anytime soon. Even with all that computing power, the numerical prediction models are not detailed enough for every weather forecasting purpose. For example, the models don’t very accurately account for the effects on weather of local landscape features such as mountains or lakes. And thunderstorms, which look pretty big when they’re coming at you, are too small and too short-lived even to show up on the big numerical models.

      And another thing about all this computing power and these state-of-the-art weather forecasting models — did I mention that they are not always accurate?

      THE BUTTERFLY EFFECT

      The atmosphere has a physical characteristic, like a personality trait, that sometimes drives forecasters to distraction, especially as they try to predict what the weather is going to be like beyond a few days.

      All disturbances in the atmosphere grow and decay. Some disturbances are big enough to be measured — and “seen” by computer models — and predicted. Other disturbances are too small. In such a chaotic system, very small disturbances can lead to big disturbances over time.

      When he discovered this characteristic of weather in the early 1960s, meteorologist Edward Lorenz at the Massachusetts Institute of Technology came up with an interesting way to describe the problem. Imagine a butterfly in the jungles of the Amazon fluttering its wings and setting in motion a subtle whirl of breeze that travels and magnifies through the atmosphere over time. Farther and farther it goes, bigger and bigger it grows. Two weeks later, this breeze results in a tornado over Kansas. This is known as the Butterfly Effect.

Should the Butterfly Effect be taken literally? No. But forecasters ever since then have taken very seriously the big idea behind it. The atmosphere’s way of doing things is on a scale that is always going to be finer, or more exacting, than the scale for the data on initial conditions that goes into the numerical forecast models. This means that, no matter how much computer power is thrown into the job, it is simply not possible to create a 100 percent accurate forecast, or a detailed weather forecast beyond about two weeks.

      “We interrupt this program …”

      Besides the regularly scheduled daily forecasts, National Weather Service offices around the United States as well as private forecasters issue special weather statements and watches and warnings and advisories appropriate to local circumstances.

      They are issued for such things as tornadoes, severe thunderstorms, floods, and winter storm conditions such as blizzards, heavy snow, ice storms or freezing rain, high winds, and dust storms.

When threats to public safety are imminent, bulletin warnings are issued — only by the National Weather Service — and a special communications network known as the Emergency Alert System is activated. When this happens, local radio and television broadcasters interrupt their regularly scheduled programs to pass them along. Every year, the National Weather Service issues between 45,000 and 50,000 severe weather warnings. Whatever the hazard, the differences between these various levels of public notices can be worth knowing.

        A Special Weather Statement often is issued as a “first alert” to the possibility of significant weather. This kind of “heads up” also is issued when forecasters see the likelihood of such things as thunderstorms with small hail, which may not be life-threatening but could make conditions temporarily hazardous.

       A Watch, such as a tornado or severe thunderstorm watch, is issued when dangerous weather conditions like lightning, large hail, and damaging winds are possible for the next several hours. It’s time to be on your toes.

       An Advisory, such as a winter weather advisory or wind advisory, is issued when conditions are not life-threatening but still worth keeping in mind. These are especially valuable to travelers in areas experiencing such hazards as snow or winds or fog. Maybe it’s time to slow down and think ahead.NOAA WEATHER RADIOTimely weather information is readily available to anyone with an FM or AM radio or television set, and most National Weather Service forecast offices maintain a website. But what happens when things go badly? If it’s emergency preparation you’re thinking of, a battery-powered NOAA Weather Radio belongs in a disaster supplies evacuation kit.This official weather emergency radio service is the only direct link the National Weather Service has to the public. It operates from about 500 transmitters in 50 states and U.S. territories on seven frequencies in the VHF band, ranging from 162.400 to 162.550 megahertz. These frequencies are outside the normal range of AM and FM broadcasts, although some manufacturers are including NOAA Weather Radio as a special feature on some receivers.Nearly every National Weather Service office operates at least one NOAA Weather Radio transmitter broadcasting weather information 24 hours a day. The average range of these transmitters is 40 miles.During severe weather, a tone alert can be activated to cause radios equipped with the alert feature to sound an audible alarm. If you’re in the market for a weather emergency radio, check to see whether it has this special tone alarm feature.

       A Warning is issued when potentially dangerous weather is possible within a matter of minutes and residents should seek shelter. A warning means no fooling around — it’s time to take some action.

       A Severe Weather Statement often follows up on a warning, to cancel it or modify the area of concern. Also, this statement might alert residents to the presence of such hazards as funnel clouds that are not expected to touch the ground.

      Water, water, everywhere …

      A major weather-related killer in the United States is flooding — not the winds of a hurricane or a tornado — and the especially dangerous circumstance of high water has its own set of National Weather Service bulletins.

      A flood is called a flash flood because it is sudden. It takes place within a few minutes or a few hours of heavy rainfall or some other event like a dam break or a river levee failure. Most flash flooding is caused by torrential rains from thunderstorms or the rains of hurricanes or tropical storms.

      Every state in the United States has been hit by flooding of one kind or another. Rivers flood sometimes in the spring when runoff from heavy rains combines with water from melting snow, although floods can happen any time of year. Along coastlines, the winds of powerful storms can generate big waves and high tides and storm surges that cause coastal flooding. The streets of every city can fill with water of urban flooding when circumstances are just right — or just wrong!

      The National Weather Service issues these special flood warnings:

        A Flash Flood Watch or Flood Watch is issued when flooding is possible. Be alert to signs of flooding and be ready to evacuate to higher ground.

       A Flash Flood Warning or Flood Warning means flooding has been reported or is imminent. It is time to act, and to act quickly, to save yourself.

       An Urban and Small Stream Advisory alerts you to the fact that flooding is occurring on some small streams or streets and low-lying areas such as underpasses and storm drains.

       A Flash Flood Statement or Flood Statement contains follow-up information about a flood

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